Methods and systems for laterally stabilized constraint of spinous processes

ABSTRACT

A spinal implant for limiting flexion of the spine includes a tether structure for encircling adjacent spinal processes. Usually, a pair of compliance members will be provided as part of the tether structure for elastically limiting flexion while permitting an extension. A cross-member is provided between the compliance member or other portions of the tether structure to stabilize the tether structure and prevent misalignment after implantation.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. patent applicationSer. No. 15/805,554 (Attorney Docket No. 48626-703.309) filed Nov. 7,2017, which is a continuation of U.S. patent application Ser. No.15/442,503 (Attorney Docket No. 48626-703.308) filed Feb. 24, 2017,which is a continuation of U.S. patent application Ser. No. 15/053,924(Attorney Docket No. 48626-703.306) filed on Feb. 25, 2016, which is acontinuation of U.S. patent application Ser. No. 13/889,581 (AttorneyDocket No. 48626-703.302 formerly 41564-703.302) filed on May 8, 2013now U.S. Pat. No. ______ , which is a continuation of U.S. patentapplication Ser. No. 13/206,339 (Attorney Docket No. WSGR 41564-703.401)filed Oct. 14, 2011, which is a divisional of U.S. patent applicationSer. No. 11/777,366 (Attorney Docket No. 41564-703.201, formerly026398-000110US) filed Jul. 13, 2007 now U.S. Pat. No. 8,029,541, whichis a non-provisional of, and claims the benefit of prior U.S.Provisional Application No. 60/862,085, (Attorney Docket No.41564-703.101, formerly 026398-000100), filed on Oct. 19, 2006; U.S.patent application Ser. No. 13/889,581 (Attorney Docket No.48626-703.302 formerly 41564-703.302) filed on May 8, 2013 now U.S. Pat.No. ______ , is also a continuation in part of U.S. patent applicationSer. No. 13/455,917 (Attorney Docket No. 41564-703.402) filed Apr. 25,2012, which is a divisional of U.S. patent application Ser. No.12/426,167 (Attorney Docket No. 41564-703.502), filed on Apr. 17, 2009,which is a continuation-in-part of PCT Application US2007/081815(Attorney Docket No. 026398-000130PC), filed on Oct. 18, 2007, whichclaims the benefit of Provisional Application No. 60/862,085 (AttorneyDocket No. 026398-000100US), filed on Oct. 19, 2006; U.S. patentapplication Ser. No. 13/889,581 (Attorney Docket No. 48626-703.302formerly 41564-703.302) filed on May 8, 2013 now U.S. Pat. No. ______,is also a continuation in part of U.S. patent application Ser. No.13/427,551 (Attorney Docket No. 41564-703.403) filed Mar. 22, 2012,which is a divisional of and claims the benefit of U.S. patentapplication Ser. No. 12/426,119 (Attorney Docket No. 41564-703.503)filed Apr. 17, 2009, which is a continuation-in-part of and claimspriority to International PCT Application No. PCT/US2007/081822(Attorney Docket No. 41564-703.601) filed Oct. 18, 2007, which claimspriority to U.S. Provisional Patent Application No. 60/862,085 (AttorneyDocket No. 41564-703.101) filed Oct. 19, 2006; U.S. patent applicationSer. No. 12/426,119 (Attorney Docket No. 41564-703.503) also is anon-provisional of, and claims the benefit of U.S. Provisional PatentApplication No. 61/158,892 (Attorney Docket No. 41564-713.101) filedMar. 10, 2009; U.S. patent application Ser. No. 13/889,581 (AttorneyDocket No. 48626-703.302 formerly 41564-703.302) filed on May 8, 2013now U.S. Pat. No. ______, is also a continuation in part of U.S. patentapplication Ser. No. 13/274,171 (Attorney Docket No. 41564-703.301)filed Oct. 14, 2011, which is continuation of International PCTApplication No. PCT/US2010/031471 (Attorney Docket No. 41564-703.606,formerly 026398-000142PC) filed Apr. 16, 2010, which claims priority toU.S. patent application Ser. No. 12/426,119 (Attorney Docket No.026398-000141US) filed Apr. 17, 2009, which is a continuation-in-part ofInternational Patent Application No. PCT/US2007/081822 (Attorney DocketNo. 026398-000140PC) filed Oct. 18, 2007, which claims priority to U.S.Provisional Patent Application No. 60/862,085 (Attorney Docket No.026398-000100US) filed Oct. 19, 2006; U.S. patent application Ser. No.12/426,119 is also a non-provisional of, and claims the benefit of U.S.Provisional Patent Application No. 61/158,892 (Attorney Docket No.026398-001300US) filed Mar. 10, 2009; U.S. patent application Ser. No.13/889,581 (Attorney Docket No. 48626-703.302 formerly 41564-703.302)filed on May 8, 2013 now U.S. Pat. No. ______, is also a continuation inpart of U.S. patent application Ser. No. 12/106,049 (Attorney Docket No.41564-703.504) filed Apr. 18, 2008, which is a continuation in part ofU.S. patent application Ser. No. 11/875,674 (Attorney Docket No.41564-703.205) filed Oct. 19, 2007, which is a non-provisional of, andclaims the benefit of U.S. Provisional Patent Application No. 60/862,085(Attorney Docket No. 41564-703.101) filed Oct. 19, 2006; the entirecontents of each of the above is incorporated herein by reference.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates generally to medical methods andapparatus. More particularly, the present invention relates to methodsand devices for restricting spinal flexion in patients having back painor other spinal conditions.

A major source of chronic low back pain is discogenic pain, also knownas internal disc disruption. Patients suffering from discogenic paintend to be young, otherwise healthy individuals who present with painlocalized to the back. Discogenic pain usually occurs at the discslocated at the L4-L5 or L5-S1 junctions of the spine (FIG. 1). Paintends to be exacerbated when patients put their lumbar spines intoflexion (i.e. by sitting or bending forward) and relieved when they puttheir lumbar spines into extension (i.e. arching backwards). Discogenicpain can be quite disabling, and for some patients, can dramaticallyaffect their ability to work and otherwise enjoy their lives.

This pain experienced by patients with discogenic low back pain can bethought of as flexion instability and is related to flexion instabilitythat is manifested in other conditions. The most prevalent of these isspondylolisthesis, a spinal condition in which abnormal segmentaltranslation is exacerbated by segmental flexion.

Current treatment alternatives for patients diagnosed with chronicdiscogenic pain are quite limited. Many patients follow a conservativetreatment path, such as physical therapy, massage, anti-inflammatory andanalgesic medications, muscle relaxants, and epidural steroidinjections, but typically continue to suffer with a significant degreeof pain. Other patients elect to undergo spinal fusion surgery, whichcommonly requires discectomy (removal of the disk) together with fusionof adjacent vertebrae. Fusion is not usually recommended for discogenicpain because it is irreversible, costly, associated with high morbidity,and of questionable effectiveness. Despite its drawbacks, however,spinal fusion for discogenic pain remains common due to the lack ofviable alternatives.

Recently, a less invasive and potentially more effective treatment fordiscogenic pain has been proposed. A spinal implant has been designedwhich inhibits spinal flexion while allowing substantially unrestrictedspinal extension. The implant is placed over one or more adjacent pairsof spinal processes and provides an elastic restraint to the spreadingapart of the spinal processes which occurs during flexion. Such devicesand methods for their use are described in U.S. Patent Application2005/02161017A1, published on Sep. 29, 2005, and having common inventorswith the present application.

As illustrated in FIG. 2, an implant 10 as described in the '017application, typically comprises an upper strap component 12 and a lowerstrap component 14 joined by a pair of compliant members 16. The upperstrap 12 is shown disposed over the top of the spinous process SP4 of L4while the lower strap 14 is shown extending over the bottom of thespinous process SP5 of L5. The compliance member 16 will typicallyinclude an internal element, such as a spring of rubber block, which isattached to the straps 12 and 14 in such a way that the straps may be“elastically” or “compliantly” pulled apart as the spinous processes SP4and SP5 move apart during flexion. In this way, the implant provides anelastic tension on the spinal processes which provides a force thatresists flexion. The force increases, typically linearly with anon-variable spring constant, as the processes move further apart.Usually, the straps themselves will be essentially non-compliant so thatthe degree of elasticity or compliance may be controlled and providedsolely by the compliance members 16.

Ideally, the compliance members 16 will remain horizontally aligned andspaced generally between the spinous processes SP4 and SP5, as showngenerally in FIG. 3. In some instances, however, the desired symmetrymay be lost if the implant structure 10 becomes circumferentiallydisplaced about the spinous processes SP4 and SP5, as shown in FIG. 4.Such displacement can affect the ability of the implant to provide auniform, symmetric elastic force to inhibit flexion of the spinousprocesses in accordance with the desired treatment.

For these reasons, it would be desirable to provide improved spinalimplants and methods for their use in inhibiting flexion in patientssuffering from discogenic pain. It would be particularly desirable ifthe improved devices would provide the desired elastic forces to thespinous processes without displacement or loss of symmetry of the deviceover time. At least some of these objectives will be met by theinventions described hereinbelow.

2. Description of the Background Art

US 2005/0216017A1 has been described above. Other patents and publishedapplications of interest include: U.S. Pat. Nos. 4,966,600; 5,011,494;5,092,866; 5,116,340; 5,282,863; 5,395,374; 5,415,658; 5,415,661;5,449,361; 5,456,722; 5,462,542; 5,496,318; 5,540,698; 5,609,634;5,645,599; 5,725,582; 5,902,305; Re. 36,221; 5,928,232; 5,935,133;5,964,769; 5,989,256; 6,053,921; 6,312,431; 6,364,883; 6,378,289;6,391,030; 6,468,309; 6,436,099; 6,451,019; 6,582,433; 6,605,091;6,626,944; 6,629,975; 6,652,527; 6,652,585; 6,656,185; 6,669,729;6,682,533; 6,689,140; 6,712,819; 6,689,168; 6,695,852; 6,716,245;6,761,720; 6,835,205; Published U.S. Patent Application Nos.2002/0151978; US 2004/0024458; US 2004/0106995; US 2004/0116927; US2004/0117017; US 2004/0127989; US 2004/0172132; US 2005/0033435; US2005/0049708; US 2006/0069447; Published PCT Application Nos. WO01/28442 A1; WO 02/03882 A2; WO 02/051326 A1; WO 02/071960 A1; WO03/045262 A1; WO 2004/052246 A1; WO 2004/073532 A1; and PublishedForeign Application Nos. EP 0322334 A1; and FR 2 681 525 A1.

SUMMARY OF THE INVENTION

The present invention provides spinal implants and methods forrestricting spinal flexion for the treatment of discogenic pain andother spinal conditions, such as spondylolisthesis, where the physiciandesires to control spinal flexion. The spinal implants comprise a tetherstructure adapted to encircle at least two spinous processes, where atleast a portion of the tether structure is adapted to elasticallyelongate to apply tension to the spinous processes as the spineundergoes flexion, i.e. as the spinous processes move apart as thepatient leans forward. The tether structure may comprise any of theparticular structures described in detail in U.S. patent applicationSer. No. 11/076,469, filed on Mar. 9, 2005, and published as US2005/0216017 A1, the full disclosure of which is incorporated herein byreference.

In particular, in the simplest embodiments, the tether structure maycomprise a single, continuous loop of material wherein all or a portionof the loop is formed of a compliant material to provide the desiredelasticity. More commonly, the tether structure will comprise one ormore band segments joined by one or more compliance members, where theband(s) are typically non-distensible and the compliance member(s)provide for the desired elasticity. In some instances, the compliancemembers may comprise spring or other elements which provide an elastictensioning force where the band member(s) are attached to opposite endsof the spring member. In other instances, the compliance members couldinclude elastomeric or other compression elements, where the bandmember(s) are attached to opposed sides of the compressive elements sothat the elasticity is provided by compression on the compressionmember.

In preferred embodiments, the tether structure will comprise a pair ofband members joined by a pair of compliance members, where an upper bandmember will be placed over the superior surface of an upper spinousprocess and the lower band member will be placed over an inferiorsurface of the lower spinous process. The compliance members will begenerally horizontally aligned across the region between the upper andlower spinous processes.

In a particular aspect of the present invention, the spinal implantswill include at least one cross-member coupled to opposed portions ofthe tether structure, where the cross member is positioned to liebetween the spinous processes when the tether structure encircles theprocesses as described above. In specific embodiments, the cross-memberwill extend between the horizontally aligned compliance members, but inother embodiments a cross-member could be coupled to other portions orcomponents of the tether structure, including the band or loop elementswhich are disposed over the spinous processes.

The cross-member(s) functions to stabilize the tether structure afterthe tether structure has been implanted over the spinous processes. Inparticular, the cross-member(s) will help maintain the symmetry of thedevice so that it does not circumferentially rotate or migrate over thespinous processes, which is a potential problem when the tether includesone or more compliance members. In addition, the cross-member(s) mayoptionally maintain the lateral spacing between the two sides of thedevice, such as between a pair of horizontally aligned compliancemembers. The cross-member(s) may further prevent or inhibit vibration orsinusoidal movement of the device which may result from dynamic and/orcyclic loading.

In addition to the stabilization functions, a cross-member may help ininitial placement and positioning of the tether structure. For example,a tether structure including a pair of horizontally aligned compliancemembers may be introduced and assembled in situ, where the cross-memberhelps establish the initial horizontal alignment between the compliancemembers. Alternatively, when no compliance members are to be used, thecross-member could itself provide for connection points for attachingupper and lower band segments. Additionally, the cross-member(s) cancreate pivot points to allow rotation or pivoting of the band relativeto the cross-member(s) as well as the other band segments.

The cross-member(s) may have a wide variety of particularconfigurations. The most common cross-member(s) will have generallyrigid structures, e.g. in the form of a rod, bar, beam, or the like. Inother instances, however, the cross-member(s) may be relativelyflexible, in some cases being in the form of a wire, ribbon, string,spring, suture, or the like. In still other configurations, thecross-member(s) may be linearly compressible, but not extensible, inorder to allow for a controlled degree of inward motion of the tetherstructure after it has been placed. In still other configurations, thecross-member(s) may be linearly non-compressible, but allow for a smalldegree of axial extension in order to prevent inward motion or intrusionof the tether structure into the region between the spinous processes.

There are also a variety of ways in which the cross-member(s) may beattached to the tether structure. Typically, the cross-member(s) will beattached to opposed compliance members (usually to housings of thecompliance member subassemblies as shown in the '017 applicationpreviously incorporated by reference), where the attachment can berigid, semi-rigid, pivotal, or the like. In a first exemplaryembodiment, the cross-member is rigidly attached to a pair of compliancemembers in a generally H-shaped configuration. In other instances, theconnections may be pivotal or non-rigid, as mentioned above. Stillfurther, the cross-member can be completely flexible which would allowfor a small degree of motion between the compliance members afterimplantation.

While most embodiments of the present invention will employ only asingle cross-member, in other embodiments two or more cross-members maybe used. For example, a pair of cross-members may be positioned betweenopposed portions of the tether structure, where an upper cross-member islocated immediately below the inferior surface of the upper spinalprocess, while the lower cross-member is positioned immediately adjacentto a superior surface of the lower spinal process. Alternatively, suchcross-member pairs may be positioned more closely to the compliancemembers, e.g. where they lie immediately above and below the compliancemembers. In still other embodiments, the cross-members may be slidablyattached to the bands or other portions of the tether structure so thatthe cross members may move in response to a force applied by the spinousprocesses or otherwise.

In all the embodiments of the present invention, it will be desirablethat the cross-member(s) provide little or no resistance to extension,i.e. motion of the adjacent spinous processes toward one another. Whenthe cross-member consists of a single rod, bar, structure, or otherflexible element extending between exposed portions of the tetherstructure, the cross-member will usually have a very small verticalheight (typically less than 6 mm, usually in the range from 1 mm to 3mm), and it is unlikely that the cross-member would contact eitherspinous process even in an extreme degree of extension, so long as thecross-member is located at a position which is equally spaced apart fromthe two spinous processes. In other instances, however, the cross-membercould have a larger cross-sectional profile which might contact eitheror both spinous processes as the spine undergoes extension. In suchcases, it is desirable that the cross-member be collapsible or otherwiseprovide minimum force against either or both processes.

Usually, the cross-member will be implanted through the interspinousligament which extends between the upper and lower spinous processes. Insuch instances, it is desirable that the cross-member itself have arelatively low profile to permit passage through the ligament withminimum trauma. Often, it will be desirable to have the cross-memberdetachable from at least one of the opposed tether structure componentsso that the cross-members or other portions of the tether structure donot need to be passed through the interspinous ligament.

In another aspect of the present invention, methods for stabilizingspinal flexion comprise positioning a continuous tether structure over apair of adjacent spinous processes on adjacent vertebrae to elasticallyrestrict flexion. The tether structure will be positioned and havemechanical properties which will elastically tension the processes whenthe processes are in flexion. In accordance with the principles of thepresent invention, opposed portions of the tether structure aremechanically coupled, usually through the interspinous ligament, inorder to stabilize the structure, particularly to inhibitcircumferential displacement of the tether structure over time.

In the exemplary embodiments, the opposed portions of the tetherstructure will comprise compliance members, and it will be thecompliance members which are mechanically coupled to stabilize thestructure in situ. Typically, the compliance members will be connectedby at least one cross-member wherein said at least one cross-member isfixably or non-fixably attached to the compliance members. In someembodiments, one end of the cross-member may be fixably attached to onecompliance member while the other member is non-fixably attached to theother compliance member. The cross-member itself may be rigid,semi-rigid, or non-rigid, and in all instances the cross-member willprovide no significant inhibition of spinal extension. Preferably, thecross-member will pass through the interspinous ligament withoutsignificant damage or compromise to its integrity.

Optionally, one or more additional tether structures may be implantedaround other pair(s) of spinous processes in the manner described above.

INCORPORATION BY REFERENCE

All publications, patents, and patent applications mentioned in thisspecification are herein incorporated by reference to the same extent asif each individual publication, patent, or patent application wasspecifically and individually indicated to be incorporated by reference.

BRIEF DESCRIPTION OF THE DRAWINGS

The novel features of the invention are set forth with particularity inthe appended claims. A better understanding of the features andadvantages of the present invention will be obtained by reference to thefollowing detailed description that sets forth illustrative embodiments,in which the principles of the invention are utilized, and theaccompanying drawings of which:

FIG. 1 is a schematic diagram illustrating the lumbar region of thespine including the spinal processes (SP), facet joints (FJ), lamina(L), transverse processes (TP), and sacrum (S).

FIG. 2 illustrates a spinal implant of the type described in US2005/0216017A1.

FIGS. 3 and 4 illustrate how the spinal implant of FIG. 2 can becomemisaligned over time.

FIG. 5 illustrates a first embodiment of a spinal implant having across-member in accordance with the principles of the present invention.

FIGS. 6A and 6B illustrate the spinal implant of FIG. 5 having a rigidcross-member.

FIGS. 7A and 7B illustrate the spinal implant of FIG. 5 having asemi-rigid cross-member.

FIGS. 8A and 8B illustrate the cross-member of FIG. 5 having an elasticcross-member.

FIG. 9 illustrates a specific embodiment of a cross-member useful in theapparatus and methods of the present invention.

FIG. 10 illustrates the cross-member of FIG. 9 in an implant.

FIG. 11 illustrates an embodiment of the present invention having a pairof cross-members.

FIG. 12 illustrates the spinal implant of FIG. 11 in an implant.

FIG. 13 illustrates an additional continuous tether structure over aspinous process on another pair of adjacent vertebrae.

FIG. 14A and 14B illustrate use of a reinforcement member on a singleconnector which does not include a compliance member, while FIGS. 15Aand 15B illustrate use of a reinforcement member on a single connectorwhich includes a compliance member.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to FIG. 5, a spinal implant 20 constructed in accordancewith the principles of the present invention comprises an upper strap22, a lower strap 24, and a pair of compliance members 26 joining theupper and lower straps. Typically, the upper and lower straps 22 and 24will be non-distensible but will be joined to the compliance members 26so that they can be expanded from a constricted configuration, as shownin broken line, when the patient's spine is in a neutral positionbetween flexion and extension, to an expanded configuration (shown infull line) when the patient's spine is in flexion. The compliancemembers 26 will provide a force which acts against the extension of thespinous processes, as generally described in prior patent applicationU.S. 2005/0216017, which has been previously incorporated herein byreference. In particular accordance with the present invention, across-member 30 extends between and joins the compliance members 26. Thecross-member 30 passes through the interspinous ligament ISL.

As shown in FIGS. 6A and 6B, the cross-member 30 may be rigid and berigidly attached to the compliance members 26 in a generally H-shapedconfiguration so that the compliance members do not shift relative toeach other even when the upper and lower bands 22 and 24 are pulledapart, as shown in FIG. 6B. Alternatively, the cross-member 30 may besemi-rigid (or semi-compliant) so that it will undergo compression whenthe upper band 22 is pulled away from the lower band 24, as shown inFIG. 7B. In a third embodiment, the cross-member 30 may be entirelyelastic, as shown in FIGS. 8A and 8B. In such instances, thecross-member 30 will allow the compliance members 26 to verticallydisplaced relative to each other by a controlled amount, as shown inFIG. 8B.

FIG. 9 illustrates an exemplary cross-member 50 which can be coupled tocompliance members 26, as shown in FIG. 10. The cross-member 50 is arigid structure which may be attached (and optionally detached) from thecompliance member during implantation of the spinal implant. Endportions 52 of the cross-member are shaped and adapted to be attached tothe cylindrical bodies of the compliance members. Other shapes andstructures for selective attachment and detachment of the cross-memberare, of course, readily available.

A pair of cross-members 60 are illustrated in FIG. 11. The cross-members60 have endpieces 62, each having a slot 64 which receives thecorresponding band 22 or 24. Cross-members 60 can thus be disposeddirectly over the upper and lower surfaces of the compliance members 26,as shown in FIG. 12. Usually, cross-members 60 will themselves becompliant in order to avoid inhibiting of extension of the spinalprocesses SP4 and SP5, as shown in broken line in FIG. 12. FIG. 13illustrates positioning at least one additional continuous tetherstructure over a spinous process on another pair of adjacent vertebrae,and mechanically coupling opposed portions of the at least oneadditional tether structure through the interspinous space.

Referring now to FIG. 14, yet another spinous process constraint system1460 comprises first and second hook-like attachment members 1462 and1464. The hook members 1462 and 1464 are connected in a C-shapedpattern, as shown in FIG. 14.

The spinous process constraint 1460 of FIG. 14 will have a tendency todeform when placed under an axial load as the spinous processes undergoa flexion causing movement in the direction of arrow 1465. Typically, aregion 1466 of the constraint will tend to bow inwardly which causes thesuperior and inferior hook members 1462 and 1464 to displace laterally,increasing the risk that they will shift from their intended positionson the spinous processes. In order to alleviate this condition, areinforcement member 1467 can be placed over a portion of the singleconnector 1463 between the hooks 1462 and 1464. The reinforcement membermay be a simple sleeve constructed from a relatively rigid material,such as a metal or rigid polymer, having a central passage which isplaced over the single connector. Other reinforcement structures wouldalso be possible. Additionally, the sleeve embodiment shown in FIG. 14Bcould be modified to be used with constraint embodiments includingcompliance members as described elsewhere in this application.

Similarly, as shown in FIG. 15, a spinous process constraint system 1570comprises first and second hook-like attachment members 1572 and 1574arranged in C-shaped pattern, generally as shown in FIG. 14, and furthercomprises compliance member 1578 attached to superior and inferiorsegments of the single connector 1576 (which is preferablynon-compliant).

The spinous process constraint 1570 of FIG. 15 can also undergodeformation when subjected to an axial load, as shown in FIG. 15A. Areinforcement assembly 1573 specifically adapted for constraints havingcompliance members 1578 is illustrated in FIG. 15B. The reinforcementassembly 1573 connects to a superior segment 1575 of the singleconnector 1576 and includes a slide rod 1571 extending toward aninferior segment 1577 of the single connector 1576. The slide rod 1571is received in a bearing structure 1579 attached to the interior segment1577 which allows the rod to translate as the segments 1573 and 1577move toward and away from each other as the spine undergoes extensionand flexion. The reinforcement assembly 1571 helps maintain the properalignment between the superior and inferior segments 1575 and 1577 toprevent the bowing and deformation illustrated in FIG. 15A

Further details and alternative embodiments of flexion limiting devices,their use, and associated instruments are disclosed in the patentapplications previously incorporated herein by reference.

While preferred embodiments of the present invention have been shown anddescribed herein, it will be obvious to those skilled in the art thatsuch embodiments are provided by way of example only. Numerousvariations, changes, and substitutions will now occur to those skilledin the art without departing from the invention. It should be understoodthat various alternatives to the embodiments of the invention describedherein may be employed in practicing the invention. It is intended thatthe following claims define the scope of the invention and that methodsand structures within the scope of these claims and their equivalents becovered thereby.

What is claimed is:
 1. A spinal implant for restricting flexion in aspine, said spinal implant comprising: a tether structure comprising anupper strap, a lower strap, and a compliance member coupling the upperstrap to the lower strap, wherein the tether structured is configured toencircle at least two spinous processes of the spine, and wherein thecompliance member elastically elongates to apply a force on the spinousprocesses to resist flexion of the spine.
 2. A method for restrictingflexion in a spine, said method comprising: providing a tether structurecomprising an upper strap, a lower strap, and a compliance membercoupling the upper strap to the lower strap; encircling at least twospinous processes of the spine with the tether structure; and resistingflexion of the at least two spinous processes.